netdev - Re: [PATCH v2] xdp: Sample xdp program implementing ip forward

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Date:   Sat, 28 Oct 2017 19:07:32 +0530
From:   Christina Jacob <christina.jacob.koikara@...il.com>
To:     David Daney <ddaney@...iumnetworks.com>
Cc:     Stephen Hemminger <stephen@...workplumber.org>,
        Christina Jacob <Christina.Jacob@...ium.com>,
        daniel@...earbox.net, Sunil.Goutham@...ium.com,
        netdev@...r.kernel.org, David Ahern <dsahern@...il.com>,
        linux-kernel@...r.kernel.org,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2] xdp: Sample xdp program implementing ip forward

On Wed, Oct 11, 2017 at 3:07 AM, David Daney <ddaney@...iumnetworks.com> wrote:
> On 10/10/2017 10:19 AM, Stephen Hemminger wrote:
>>
>> On Tue, 10 Oct 2017 12:58:52 +0530
>> Christina Jacob <christina.jacob.koikara@...il.com> wrote:
>>
>>> +/* Get the mac address of the interface given interface name */
>>> +static long *getmac(char *iface)
>>> +{
>>> +       int fd;
>>> +       struct ifreq ifr;
>>> +       long *mac = NULL;
>>> +
>>> +       fd = socket(AF_INET, SOCK_DGRAM, 0);
>>> +       ifr.ifr_addr.sa_family = AF_INET;
>>> +       strncpy(ifr.ifr_name, iface, IFNAMSIZ - 1);
>>> +       ioctl(fd, SIOCGIFHWADDR, &ifr);
>>> +       mac = (long *)ifr.ifr_hwaddr.sa_data;
>>> +       close(fd);
>>> +       return mac;
>>
>>
>> Always check return value of ioctl.
>> You are assuming sizeof(long) > 6 bytes.
>> Also the byte order.
>
>
>
> Also:
>
> Returning the address of a local variable (ifr.ifr_hwaddr.sa_data), and then
> dereferencing it outside of the function is not correct.
>
> The casting of the char sa_data[] to a long * may cause alignment faults on
> some architectures.  The may also be endinaness issues depending on how the
> data are manipulated if you pack all those chars into a long.
>
> If we think that a MAC address is char[6], then it may be best to define the
> data structures as such and manipulate it as an array instead of trying to
> pack it into a long.

How do I feed the MAC address to xdp.data ? Is it ok to do a manual
leftshift + biwise and  for the purpose?

>
> Keep working on this though, this program will surely be useful.
>
> David Daney